Generally speaking, two types of rectifying schemes may be used in the secondary side of a flyback converter: (1) non-synchronous rectifying which requires a diode (FIG. 1A) and (2) synchronous rectifying which rectifies the current through controlling on/off of a synchronous rectifier, e.g. an N-MOSFET (FIG. 1B). The voltage-current characteristic is plotted in FIG. 1C, for a diode (curve 12) and a synchronous rectifier (curve 11). In practical applications, the work area of a low power flyback power converter always falls into the shadowed area. The resistance of a synchronous rectifier is less than that of a diode in the area because curve 11 is always above curve 12. So, compared with using a diode, a scheme that uses a synchronous rectifier is more preferable because of less power waste and better efficiency. Synchronous rectifiers have thus found increasingly wide applications in devices sensitive to power efficiency, such as laptop adapters, wireless equipment, LCD power management modules and so on.
There are two methods for driving a synchronous rectifier. One method controls on/off of the synchronous rectifier based on the switching signal of the primary side switch. The drawback of this method is high cost for its relatively complicated structure. Furthermore, when light load or no load occurs, the control result is not always reliable.
A better method is independent on the primary side switching signal, but instead utilizes the characteristic of the body diode in a MOSFET. The method simulates the working function of a Schottky Diode, where the MOSFET will be turned on at forward-biased voltage and turned off at reversed-biased voltage. FIG. 2A shows a flyback power converter with a secondary synchronous rectifier Q1, arranged in the low side of the converter, with its source terminal connected to the ground terminal. FIG. 2B shows the waveform of Vds, the drain to source voltage of Q1. Vthr1 and Vthr2 are threshold voltages predetermined both lower than 0V and Vthr2 is lower than Vthr1 but higher than −Vcon, and Vcon is the voltage across body diode of Q1. Signal Vg drives gate of Q1 to turn it on when Vds is lower than Vthr2 and turn it off when Vds is higher than Vthr1.
When rectifier Q1 is off, switch A turns on with a direct current voltage Vin applied on the primary side of transformer T1, which inducts a voltage on the secondary side of T1 and makes body diode of Q1 reversed-biased. Vds can be given by Vds=(N2/N1)*Vin+Vout, here N1 and N2 standing for the winding turns of the primary and secondary side of T1 respectively. At time t=t1, switch A is cut off, leading to a reversed voltage induced across the secondary side of T1, so energy can be supplied to load through the forward-biased body diode of Q1. Forward-biasing of body diode makes Vds drop to a lower level equal to −Vcon, which is lower than Vthr2, so a driving signal is applied to gate of Q1 and turns it on. When Q1 enters into the equilibrium state, Vds can be expressed as Vds=−Rdson*I, in which I is source-drain current of Q1 and Rdson is the on-resistance of Q1. With the source-drain current decays, Vds rises gradually. At time t=t2, Vds rises to higher than Vthr1, which turns Q1 off. With the repetition of switching of switch A, the whole process repeats.
The drawback of this method is that it may cause false triggers under some conditions. Referring to FIG. 2B, after the time t1 when Q1 is turned on, there is a short period during which Vds fluctuates rapidly. If Vds rises to a value higher than Vthr1 as point A in FIG. 2B, Q1 will be turned off falsely. And after time t2 when Q1 is turned off, if body diode of Q1 is turned on again for the residual current, Vds may drop to a value lower than Vthr2 as point B in FIG. 2B, Q1 will be turned on falsely.